This invention relates to an oversampling analog-to-digital (A/D or A-to-D) converter for sampling an input analog signal at an oversampling frequency into an output digital signal.
An oversampling analog-to-digital converter is ordinarily manufactured by using a very large scale integration techology and is useful in digital communication. With an oversampling analog-to-digital converter, an input analog signal of an input signal frequency bandwidth is sampled into an output digital signal at an oversampling frequency which is much higher than the input signal frequency bandwidth, for example, higher than one hundred times the input signal frequency bandwidth.
An oversampling analog-to-digital converter is disclosed in a report contributed jointly by Akira Yukawa, Rikio Maruta, Yuuichi Kawakami, and Kyuichi Hareyama as Report No. 563 in Japanese and with a title in English to "Syowa 59-nendo Densi Tusin Gakkai Tusin Bumon Zenkoku Taikai Ronbunsyu" (Reports for 1984 General Meeting of Communication Branch of the Institute of Electronics and Communication Engineers of Japan), under the title of "An Oversampling A/D Converter Circuit Configuration for Digital Codec IC." A similar and other oversampling analog-to-digital converters are discussed in a paper contributed jointly by Akira Yukawa, Rikio Maruta, and Kenji Nakayama to Proceedings of IEEE International Conference on Acoustics, Speech, and Signal Processing held March 1985, pages 1400 to 1403, under the title of "An Oversampling A-to-D Converter structure for VLSI Digital CODEC's."
According to the Yukawa et al. report and the Yukawa et al. paper, an oversampling analog-to-digital converter has a converter input terminal supplied with an input analog signal. An input switched capacitor is connected to the input terminal and is for sampling the analog signal into output electric charges. An integrator is for integrating input electric charges into an integrated signal, which is quantized by a quantizer into an output digital signal. Control logic is used in producing a predetermined number of control signals in response to the digital signal. A capacitor array is connected to a reference voltage source and controlled by the control signals to produce controlled electric charges. A connection is used in combining the input switched capacitor and the capacitor array to the integrator to combine the output electric charges and the controlled electric charges into the input electric charges.
The oversampling analog-to-digital converter of the Yukawa et al report or the Yukawa et al. paper operates very well with the analog signal of an input signal dynamic range which is approximately as wide as a power source voltage used in putting the converter into operation. It should, however, be pointed out in the manner which will later be described more in detail that the integrator is not operable at a high speed in the converter of Yukawa et al. This imposes an undesirable restriction on the oversampling frequency. If the integrator is somehow adapted to a high-speed operation, analog-to-digital conversion imprecise. Furthermore, the converter becomes requires greatly increased power.